Gas sampling

ABSTRACT

Apparatus for sampling fluid flow in a conduit comprising a sampling probe having an aerodynamically shaped nozzle and a double static tube axially located within the nozzle and extending beyond its mouth to terminate in a closed, aerodynamically contoured end, the outer wall of the extended portion of the tube being provided with first and second orifices to communicate with the bore of the tube and the annular passage of the tube remote from the extended portion operatively communicating with a pressure sensing device to sense the static pressure of the fluid flow outside the nozzle and the static pressure of the fluid flow entering the mouth of the nozzle.

United States Patent Louis 1 June 27, 1972 54] GAS SAMPLING 2,452,22410/1948 Collett, Jr

[72] Inventor: Rodney Brian Louis, Johannesburg, Re-

33:52: et pub! Smth Afnca 3,050,996 8/1962 Henderson ..73/182 [73]Assignee: African Explosives and Chemical Industries Limited PrimaryExaminer-Louis R. Prince Assistant Examinerwilliam A. Henry, II [22]Flled' 1970 Attorney-Cushman, Darby and Cushman Appl. No.: 63,528

[57] ABSTRACT Apparatus for sampling fluid flow in a conduit comprisinga sampling probe having an aerodynamically shaped nozzle and a doublestatic tube axially located within the nozzle and extending beyond itsmouth to terminate in a closed, aerodynamically contoured end, the outerwall of the extended portion of the tube being provided with first andsecond orifices to communicate with the bore of the tube and the annularpassage of the tube remote from the extended portion operativelycommunicating with a pressure sensing device to sense the staticpressure of the fluid flow outside the nozzle and the static pressure ofthe fluid flow entering the mouth of the nozzle.

5 Claims, 2 Drawing Figures P R ESSURE SENSING PATENTEDJum I9723.672.225

EEEEJ' P'A'TENTEDJum 1972 3. 672,225

sum 2 OF 2 FIGURE 2.

/l diw GAS SAMPLING This invention relates to apparatus for samplingfluid such as dust or mist-laden effluent gases from chemical orindustrial plants, and particularly to an isokinetic sampling probe usedin sampling these gases.

Sampling probes used for determining the dust or mist in stack gasesoften produce unreliable results or are difficult to handle and, in viewof present legislation covering atmospheric pollution in many countries,hinder the routine analyses of effluent gases which have to be carriedoutin view of this legislation. It is important, therefore, thatapparatus be of simple construction and be easy to manipulate.

The conventional apparatus for sampling effluent gases comprises asampling probe having a shaped nozzle, which is inserted into the streamto be sampled. The gas is drawn by means of a vacuum pump through theprobe and a sampling train consisting of a filter, a cyclone orelectrostatic precipitator, followed by a condenser, catch-pot and arotameter.

' The particulate matter is trapped in the filter and water vapor, ifpresent, is condensed in the condenser.

The gas sample must be collected under isokinetic conditions, that is,the velocity of the gas entering the nozzle of the probe should be thesame as the velocity of the gas stream from which the sample is taken,otherwise the sample is not truly representative of the effluent stream.To ensure these conditions, the velocity of the gas stream must becontinuously monitored and, when deviations occur, the sampling rate hasto be adjusted accordingly. This is normally done by using Pitot tubesand the readings obtained are converted to velocities by means of sliderules or nomographs supplied with the apparatus. At the end of thesampling period, the weight of particulate matter collected in thefilter is determined and related to volume of gas sampled to obtain thecontaminate concentration in the stack gas.

An improved apparatus for sampling fluid uses a so-called Null PointProbe, that is, a probe having a specially designed nozzle through whichthe static pressures inside and outside the nozzle can be measured bymeans of orifices situated in the inner and in the outer walls of thenozzle construction. By maintaining these inside and outside pressuresequal, isokinetic conditions are obtained. The use of these nozzlesobviates the measurement individually of the velocity of the gas streamand of the sampling rate. Nevertheless, it has been found that thestatic pressures measured via the sensing orifices on the inside andoutside of the nozzle are influenced by the nozzle shape. The shape ofthe nozzle causes eddy and vortex formations on the outside, as well asa pressure loss on the inside of the nozzle. Consequently, erraticresults are obtained when using these nozzles.

A further disadvantage is that the nozzles are not readilyinterchangeable so that, at high or low gas velocities, the samplingrates are either very high or extremely low respectively.

It is the principal purpose of the present invention to provide a nullpoint sampling probe, which largely or wholly eliminates thedifficulties encountered in the prior art.

We have now discovered that these undesirable influences afiectingstatic pressures because of the shape of the nozzle may be avoided bysensing the static pressures outside, and at the entrance to, thenozzle. This advance in the art is achieved by means of a double tubecomprising two concentric tubes which provide an axial bore and anannular passage surrounding the wall of that bore. This new feature,which is introduced as part of a null point probe, we designate by theexpression double static tube.

Accordingly, we provide apparatus for sampling fluid flow in a conduitcomprising a sampling probe having an aerodynamically shaped nozzle anda double static tube as defined herein axially located within the nozzleand extending beyond its mouth to terminate in a closed,aerodynarnically contoured end, the outer wall of the extended portionof the tube being provided with first and second orifices to communicatewith the bore and with the annular passage respectively of the tube andthe opposite end of the bore and the annular passage of the tube remotefrom the extended portion operatively communicating with a pressuresensing device to sense the static pressure of the fluid flow outsidethe nozzle and the static pressure of the fluid flow entering the mouthof the nozzle.

The first orifices of the double static tube for sensing the pressureoutside the nozzle are located, preferably, at a distance of at least 13tube diameters from the streamlined end of the tube to avoid any eddy orvortex formation within the fluid, which may be caused during its flowpast the end of the tube.

The second orifices for sensing the pressure of the fluid flow enteringthe nozzle are located, preferably, on or adjacent the outside of theplane of the mouth of the nozzle. This position has been determined toavoid a pressure drop within the nozzle. Additionally, these secondorifices should be at a distance of at least 21 tube diameters from thestreamlined end of the tube to avoid possible interference caused by thefirst orifices.

The pressure sensing orifices in the tube have a diameter, preferably,of not more than one tenth of the external diameter of the tube.

The nozzle, which is part of the probe, may be replaced withoutdifliculty, since the pressure sensing devices are not located in thebody of the nozzle.

An embodiment of the apparatus will be described with reference to theaccompanying drawings, in which FIG. 1 is a cross section of theassembled sampling probe, and

FIG. 2 is an enlarged cross section of the double static tube, formingpart of the probe.

In FIG. 1, the sampling probe l is shown with detachable nozzle 2 and adouble static tube 3. The first pressure sensing orifices 4 and thesecond pressure sensing orifices 5 in the wall surrounding the centralpassage of the extended portion of the double static tube 3 are incommunication with a pressure sensing device via manifold 6 and tubes 7.

FIG. 2 illustrates the detailed construction of double static tube 3 andshows the location of the pressure sensing orifices 4 and 5 in relationto the streamlined end 9 of the tube. The frontal portion 8 of the tubemay be made of solid material.

The apparatus is used in conjunction with a standard filter, cyclone orelectrostatic precipitator and a vacuum pump. Any sensitive sensingdevice may be used to measure the pressure difference between the firstand second sensing orifices.

The size of the nozzle used is dependent on the velocity of the gasstream and, preferably, is selected to give a sampling flow rate ofbetween 5 I and 6 cubic meters per hour under isokinetic conditions, sothat the sampling periods for collecting a representative sample arebetween 5 and 10 minutes.

Water vapor, if present in the gas flow, may condense in the cold partsof the sampling probe and a heated mantle 10 may be provided aroundthese parts to prevent this condensation.

The probe was tested in an experimental duct with sampling velocitiesranging from 5 to 25 meters per second and four sizes of nozzles wereused to cover this range.

The sampling flow rate was measured on an accurate rotameter and thedifference between this measured flow rate and the flow rate calculatedfrom the velocity and the nozzle means defining a closed,aerodynamically contoured end;

said double static tube including means defining a longitudinal boreterminating short of said closed, aerodynamically contoured end;

a means defining a first orifice communicating the exterior of thedouble static tube, toward said end from said mouth, with said bore;

means defining an annular passage in said double static tube surroundingsaid bore, but being separated therefrom;

means defining a second orifice communicating the exterior of the doublestatic tube, toward said end from said mouth, with said annular passage;

said first and second orifices being axially displaced from one anotheralong said double static tube;

a pressure sensing device for sensing the static pressure of the fluidflow outside the nozzle and the static pressure of the fluid flowentering the mouth of the nozzle;

conduit means independently communicating said bore and said annularpassage to said pressure sensing device remotely of and upstream of themouth of the nozzle as inputs to said pressure sensing device.

2. Apparatus as claimed in claim 1 in which the first orifice forsensing the static pressure outside the nozzle is located at a distanceof at least 13 tube diameters from the end of the tube.

3. Apparatus as claimed in claim 1 in which the second orifice forsensing the static pressure inside the nozzle is located no furtherupstream than a radial plane across the mouth of the nozzle at adistance of at least 2! tube diameters from the end of the tube 4.Apparatus as claimed in claim 1 in which each of the pressure sensingorifices in the tube has a diameter of not more than one tenth of theexternal diameter of the tube.

5. Apparatus as claimed in claim 1 in which the nozzle of the probe isdetachably connected thereto, so as to be interchangeable with othernoules of different dimensions and shapes.

* t F l

1. Apparatus for sampling fluid flow in a conduit, comprising: asampling probe, including means defining a tubular nozzle having meansdefining a mouth; a double static tube extending axially within saidnozzle and extending out past the mouth thereof to terminate in meansdefining a closed, aerodynamically contoured end; said double statictube including means defining a longitudinal bore terminating short ofsaid closed, aerodynamically contoured end; a means defining a firstorifice communicating the exterior of the double static tube, towardsaid end from said mouth, with said bore; means defining an annularpassage in said double static tube surrounding said bore, but beingseparated therefrom; means defining a second orifice communicating theexterior of the double static tube, toward said end from said mouth,with said annular passage; said first and second orifices being axiallydisplaced from one another along said double static tube; a pressuresensing device for sensing the static pressure of the fluid flow outsidethe nozzle and the static pressure of the fluid flow entering the mouthof the nozzle; conduit means independently communicating said bore andsaid annular passage to said pressure sensing device remotely of andupstream of the mouth of the nozzle as inputs to said pressure sensingdevice.
 2. Apparatus as claimed in claim 1 in which the first orificefor sensing the static pressure outside the nozzle is located at adistance of at least 13 tube diameters from the end of the tube. 3.Apparatus as claimed in claim 1 in which the second orifice for sensingthe static pressure inside the nozzle is located no further upstreamthan a radial plane across the mouth of the nozzle at a distance of atleast 21 tube diameters from the end of the tube.
 4. Apparatus asclaimed in claim 1 in which each of the pressure sensing orifices in thetube has a diameter of not more than one tenth of the external diameterof the tube.
 5. Apparatus as claimed in claim 1 in which the nozzle ofthe probe is detachably connecTed thereto, so as to be interchangeablewith other nozzles of different dimensions and shapes.